The source-to-sink system in continental rift lacustrine basins of the Nantun Formation in X Sag of the Tamtsag Basin, Mongolia

Abstract

The strata of the first member of the Nantun Formation constitute the primary targets for oil and gas exploration and development in X sag of the Tamtsag Basin. However, the distribution pattern of sand bodies within these strata remains poorly understood due to the complex interplay of factors such as provenance supply, boundary fault growth and evolution, and sediment transport pathways, which have led to diverse source-to-sink (S2S) systems. This study aims to provide a detailed characterization of these S2S systems. The results indicate the following: (1) Activity of the boundary faults within the first member of the Nantun Formation increased progressively, controlling paleovalley development and migration of the subsidence center and depocenter; (2) Three distinct S2S systems are identified in the study area: the “transverse drainage - fan delta” system, the “transverse drainage - braided delta” system, and the “axial drainage - braided delta” system, with their spatial distributions exhibiting significant inheritance; (3) Evolution of the S2S systems is governed by growth of the sag-controlling boundary faults and variations in provenance supply intensity.

Introduction

The source-to-sink (S2S) system delineates the genetic linkages between sediment production, transport dynamics, and depositional architecture within sedimentary basins^1,2,3,4,5. These relationships form the foundation for understanding sediment routing mechanisms, compositional differentiation,

and reservoir sandstone distribution prediction. S2S analysis has significantly advanced this understanding, notably within continental rift lacustrine basins, where its methodologies are increasingly applied to hydrocarbon exploration^6,7,8,9.

Investigating S2S systems in continental rift lacustrine basins is scientifically significant but methodologically challenging. These basins preserve complex sediment-dispersal records and host substantial hydrocarbon reserves, making their analysis essential for advancing tectonic-sedimentary theory^{10,11,12,13,14,15}. Their multiphase tectonic evolution—characterized by multistage fault reactivation, differential subsidence, and multifactorial controls—generates diverse paleogeographic configurations that govern S2S architectural heterogeneity^{11,16,17,18,19}. While previous studies have proposed sediment-dispersal models for such basins^20,21, providing a foundation for sedimentary environment analysis and reservoir prediction, these models inadequately constrain key developmental controls. Critical factors include: (1) formation and evolution of boundary faults, (2) paleogeography of provenance areas, (3) sediment-supply intensity, and (4) sediment-transport pathways. Furthermore, analysis of the spatiotemporal coupling among these controlling factors remains particularly limited^21,22,23.

The Tamtsag Basin—a classic continental rift lacustrine basin in eastern Mongolia—hosts the hydrocarbon-rich X Sag depocenter in its eastern sector. Within this basin, the first member of the Nantun Formation (K₁n₁), the primary oil-bearing interval, exhibits complex reservoir sandstone distribution. This complexity arises from the interplay of: (1) differential boundary-fault activity, (2) multidirectional sediment sources, (3) depocenter migration through time, and (4) depositional-system heterogeneity^24,25,26. Previous studies have focused predominantly on the adjacent Tanan Depression, leaving the X Sag’s S2S system poorly constrained. To address this gap, we integrate high-resolution 3D seismic data with dense well-log datasets to reconstruct the S2S framework. This integrated approach enables robust seismic-sedimentological facies characterization, optimizing hydrocarbon exploration strategies.

The objectives of this study are threefold: (1) To delineate the boundary-controlling faults of the depression and characterize their growth and evolutionary history; (2) To utilize seismic geomorphology, within distinct geomorphic settings, to determine sediment source directions, transport pathways, sedimentary system types, and sand body distribution patterns, enabling the construction of a S2S element coupling model; (3) To summarize the evolutionary processes of the identified S2S systems and establish the controlling factors for each system type.

The exploratory approach employed in this study provides valuable insights for hydrocarbon exploration in continental rift basins with analogous geological settings, such as the Erlian Basin, Genhe Basin, and East Gobi Basin. Moreover, the findings not only advance the application of S2S system analysis within continental lacustrine rift basins but also help reduce exploration risks in these regions.

Geological setting

The Tamtsag Basin, situated in Dornod Province, eastern Mongolia, covers an area of approximately 35,400 km²²⁷. This Mesozoic-Cenozoic continental rift basin formed during the collision between the Sino-Korean Block and the Siberian Plate¹⁷. The basin is flanked by the Mongol-Okhotsk Suture Zone to the west and the Greater Khingan Fold Belt (Inner Mongolia, China) to the east^28,29,30. Under NW-SE oriented Cretaceous tectonic extension, the basin developed a dominant NE-SW structural trend (Fig. 1A)¹⁶.

Fig. 1

Structural framework of the study area. (A) Tectonic map of the Hailar-Tamtsag Basin (pink box outlines the X Sag). (B) Seismic survey extent and structural framework of the study area. The orange area represents the extent of the denuded western catchment zone. (C) Stratigraphic cross-section across the X Sag.

The Cretaceous stratigraphic evolution of the Tamtsag Basin comprises two principal phases: syn-rift extension and post-rift thermal subsidence³¹. Deposition commenced with the Tongbomiao Formation (K₁t, 145–139.4 Ma) during the early syn-rift stage. This was succeeded by the Nantun Formation (K₁n, 139.4–133.9 Ma) during the peak rifting phase, the Damoguaihe Formation (K₁d, 133.9–130.8 Ma) during late syn-rift activity, and the Yimin Formation (K₁y, 130.8–100.5 Ma) during the post-rift thermal subsidence stage²⁴. These stratigraphic units correlate with seismic reflectors T₃, T₂₃, T₂₂, T₂, and T₀₄^29,32 (Fig. 1).

The Tamtsag Basin comprises five sub-depressions: the Tanan, Nanbeir, Bayangebi, Chanan, and Sangbur depressions. Of these, the Nanbeir Depression is a key hydrocarbon-bearing unit within the central fault zone. This NE-trending composite half-graben is characterized by alternating sags and uplifts and is bounded to the east by the Beierbuynor Uplift, to the west by the Sudert Uplift, to the north by the Beier Depression, and to the south by the Tanan Depression³². Structurally, the Nanbeir Depression consists of the Western Sub-sag, Central Uplift, Eastern Sub-sag, and Southern Buried Hill Zone. Situated in the northern Eastern Sub-sag, the X Sag is the basin’s most prolific oil-bearing depocenter³³. The geometry of the X Sag, controlled by the western boundary fault, exhibits a west-faulted, east-onlap configuration³⁴.

The Tongbomiao Formation is predominantly characterized by fan-delta and lacustrine facies, comprising variegated sandy conglomerate, grey tuffaceous sandstone, siltstone, tuff, and thin-bedded mudstone. Within the Tamtsag Basin, the K₁n₂ serves as a significant source rock, composed predominantly of dark grey to greyish-black mudstone deposited in deep-lake to semi-deep-lake subfacies. The K₁n₁ represents one of the principal oil-producing intervals in the Tamtsag Basin, encompassing sedimentary systems including fan-delta, braided delta, lacustrine, and sublacustrine fan facies. The Damoguaihe Formation is predominantly composed of braided delta and lacustrine facies. Finally, the Yimin Formation is characterized primarily by lacustrine and fluvial facies²⁶.

Exploration and development results indicate that hydrocarbons are predominantly concentrated within the K₁n₁ interval of the X sag. The K₁n₁ interval comprises four high-frequency sequences: designated as Oil Zone 0 (N10), Oil Zone I (N1I), Oil Zone II (N1II), and Oil Zone III (N1III)^34,35.

Materials and methods

Datasets

Seismic data

This study utilizes a 3D time-migrated seismic survey covering an area of approximately 334 km², which fully encompasses the study area. The dataset, acquired by the CNPC Tamtsag Oilfield, has inline and crossline spacings of 250 m, an effective frequency bandwidth from 7 to 76 Hz, and a dominant frequency of 38 Hz; these specifications enable subsurface interpretation at the required resolution. Seismic interpretation reveals that the structural top of the K₁n₁ interval forms a gently westward-dipping monocline, bounded by two NNE-striking normal faults (F1 and F2) along the western margin.

Well and core data

Well log data from a total of 520 wells, including 67 exploration wells and 453 development wells, were analyzed. These logs contained gamma-ray (GR) and resistivity curves. Furthermore, grain size data from 50 wells, detrital component analyses from 13 wells, and core photographs covering 115 m from 7 wells were systematically processed. Additionally, heavy mineral assemblages from 16 wells provided constraints on sediment provenance directions using stable mineralogical signatures.

Methods

Analysis of boundary fault activity

The throw-distance profile method was employed to quantify the spatio-temporal evolution of the boundary faults. Despite extensive erosion in the footwall of the boundary faults, the intact Tongbomiao Formation seismic horizon (T₃) enabled reliable throw calculations (Fig. 2). The workflow comprised the following steps:

Fig. 2

Fault throw-displacement curves of the boundary faults. (A) Present-day. (B) Fault throw-displacement curves of each oil zone during the sedimentary period of T₂₂. (C) Fault throw-displacement curves of each oil zone during the sedimentary period of T₂₃. (D) Fault throw-displacement curves of each oil zone during the sedimentary period of T₂₃₁. (E) Fault throw-displacement curves of each oil zone during the sedimentary period of T₂₃₂.

• (1) Extracting two-way time (TWT) depths at the intersection of the hanging wall with syn-rift seismic horizons.

• (2) Measuring TWT depths at the T₃-hanging wall intersection.

• (3) Calculating vertical throw as the TWT difference between syn-rift horizons and T₃.

• (4) Constructing throw-distance curves for each horizon.

Sedimentary provenances analysis

Provenance directions were constrained using integrated approaches. First, QFL (Quartz-Feldspar-Lithic) ternary diagrams³⁶ depict the relative proportions of quartz (typically derived from recycled orogenic belts), feldspar (commonly sourced from crystalline basement uplifts), and lithic fragments (often indicative of dissected magmatic arcs), enabling the distinction of sediments derived from different tectonic settings. Second, stable heavy mineral assemblages provide constraints on provenance directions. Third, contour maps of net-to-gross ratio (N/G) trace sediment dispersal trends under consistent accommodation/supply (A/S) conditions.

Seismic geomorphology

Seismic geomorphology, a subdiscipline of seismic sedimentology, utilizes 3D seismic reflection patterns and horizon-based imaging techniques to reveal paleotopography and depositional system architectures^37,38. Key methodologies include spectral decomposition and RGB (Red-Green-Blue) blended attribute analysis, both of which effectively delineate sediment dispersal patterns^38,39,40,41. Compared to conventional amplitude-based attributes, RGB blended slices provide superior spatial resolution and enhanced sensitivity to lithological anomalies, making it possible to resolve finer-scale geomorphic features critical for reservoir characterization^42,43,44.

Spectral decomposition generated frequency-dependent amplitude volumes for 40 Hz (low-frequency), 70 Hz (mid-frequency), and 100 Hz (high-frequency). These volumes were then mapped to an RGB color scheme to create a blended attribute slice: 40 Hz to blue, 70 Hz to green, and 100 Hz to red, following standard seismic geomorphology workflows. The resulting RGB slices, representing key depositional time intervals of the reservoir units, were integrated with well-log facies characteristics to constrain and achieve high-resolution delineation of the depositional systems (Fig. 9).

Result

Boundary fault characterization of the X Sag

The evolution of large-scale faults generally progresses through three distinct phases: isolated segment growth, soft linkage, and hard linkage^45,46,47. To delineate the spatio-temporal evolution of the boundary faults controlling the X Sag during the K₁n₁ interval, throw-displacement profiles were constructed for successive depositional stages. These profiles were refined using the maximum throw subtraction method (Fig. 2), which enabled a quantitative reconstruction of the fault growth histories.

Two boundary faults, F1 and F2, control the structural framework of the X Sag. The northern terminus of F1 intersects the southern terminus of F2 near seismic line L3275, justifying their combined analysis on a single throw-displacement plot (Fig. 2). Both faults exhibit progressive throw amplification from the T₂₃₃ to T₂₃ horizons. F1 consistently demonstrates greater throw magnitudes than F2 across all stages, indicating higher fault activity.

Fault F2, situated in the northern part of the study area, exhibits progressive along-strike lengthening over time. During the N1III-N1I intervals, it manifested as an isolated minor fault. In contrast, Fault F1 displays segmented growth patterns, with a prominent segmentation point at seismic line L2875, dividing it into northern (F1-1) and southern (F1-2) segments. Both segments strike NE-SW, with maximum throws of 1402 ms (F1-1) and 1320 ms (F1-2), respectively. The C-shaped displacement profile of F1, characterized by peak throws near its center that taper toward the tips, suggests an evolutionary process involving initial isolated segment growth, followed by lateral propagation and eventual linkage with F2 during the N10 interval. Notably, syn-sedimentary fault activity typically induces hanging-wall thickening. However, the absence of significant thickness variations perpendicular to the boundary faults during the N1III interval implies tectonic quiescence at this stage.

Sedimentary provenances analysis

Reservoir petrology, stable heavy mineral assemblages, and the net-to-gross ratio collectively serve as robust proxies for sediment provenance^48,49. In this study, these three approaches are integrated to reconstruct the provenance system of the target interval.

Petrographic analysis

QFL (Quartz-Feldspar-Lithic) ternary diagrams, which correlate sandstone compositions (quartz, feldspar, lithic fragments) with tectonic settings (e.g., craton interior, recycled orogen, magmatic arc), were employed to delineate the provenance characteristics of the K₁n₁ interval^48,50.

During the N1III depositional period, samples plot within the transitional arc, undissected arc, and dissected arc fields (Fig. 3D). The Well W74, located in the northern part of the study area, exhibits mixed transitional and dissected arc signatures, whereas the Well W25-3, situated in the southern part, clusters within the undissected arc field. These observations suggest the presence of two distinct provenance terrains, with more pronounced tectonic activity in the northern region.

Fig. 3

Dickinson ternary plot for provenance discrimination in the K₁n₁ of Sag X. (A) N10, (B) N1I, (C) N1II, (D) N1III.

During the N1II depositional period, transitional arc signatures predominate, with the exception of the Well W36-4 in the northern part of the study area, which exhibits an undissected arc affinity (Fig. 3C). This pattern suggests enhanced provenance activity from the south, consistent with the trends observed during the N1III interval.

During the N1I depositional period, both transitional and undissected arc domains are represented (Fig. 3B). The Well W20-3 (northwest) and the W77 (east) exhibit undissected arc signatures, indicating relatively weaker tectonic activity in the northwestern and eastern source areas compared to the southwest.

During the N10 depositional period, all samples cluster within the transitional arc field (Fig. 3A), indicating intensified tectonic activity in the provenance regions. This pattern is consistent with the trends of boundary fault activation observed in “Boundary fault characterization of the X sag” and Fig. 2.

The QFL framework indicates that the K₁n₁ provenance system was consistently located within a magmatic arc tectonic setting, which can be further divided into transitional, dissected, and undissected subdomains. The upward-increasing transitional arc signatures from N1III to N10 (Fig. 3) suggest progressive tectonic intensification, consistent with the evolution of fault activity.

Heavy mineral assemblage

Stable heavy mineral assemblages, which are resistant to mechanical and chemical degradation during transport, serve as reliable indicators of provenance terrains^51,52,53. In the K₁n₁ interval of the X Sag, eight stable heavy mineral species were identified: zircon, titanite, rutile, garnet, apatite, tourmaline, magnetite, and leucoxene (Fig. 4). These assemblages exhibit pronounced spatial partitioning across different oil zones.

Fig. 4

Proportional diagrams of stable heavy mineral assemblages. (A) N10, (B) N1I, (C) N1II, (D) N1III. AHM, Assemblage of heavy minerals.

During the deposition of N1III, six distinct stable heavy mineral associations were identified (Fig. 4D): ① Zircon-apatite (AHM1), ② Apatite-zircon (AHM2), ③ Zircon-magnetite-garnet (AHM3), ④ Zircon-apatite-leucoxene (AHM4), ⑤ Zircon-apatite-magnetite (AHM5), ⑥Magnetite-zircon (AHM6).

During the deposition of N1II, six stable heavy mineral associations were identified (Fig. 4C): ① Zircon-apatite-leucoxene (AHM7), ② Apatite-zircon-garnet (AHM8), ③ Zircon-apatite-leucoxene-garnet (AHM9), ④ Leucoxene-zircon (AHM10), ⑤ Zircon-magnetite-apatite (AHM11), ⑥ Zircon-apatite (AHM12).

During the deposition of N1I, five stable heavy mineral associations were identified (Fig. 4B): 0① Zircon-apatite-leucoxene (AHM13) with subordinate magnetite, ② Zircon-magnetite-apatite (AHM14), ③ Zircon-leucoxene-apatite (AHM15), ④ Zircon-apatite-magnetite (AHM16), ⑤ Zircon-apatite-magnetite with minor garnet/leucoxene (AHM17).

During the deposition of N10, five stable heavy mineral associations were identified, exhibiting spatial variability across different sub-sags of the study area (Fig. 4A): ① Western Sag: Zircon-apatite (AHM18), ② Western Sag: Zircon-apatite-leucoxene (AHM19), ③ Western Sag: Zircon-apatite-leucoxene-magnetite (AHM20), ④ Southern Sag: Zircon-apatite-leucoxene-garnet (AHM21), ⑤ Northeastern Sag: Zircon-apatite (AHM22).

This temporal evolution reveals six discrete provenance inputs during the N1III-N1II intervals, which reduce to five dominant sources in the N1I-N10 intervals. This pattern reflects dynamic sediment routing reorganization.

Net to gross ratios

The net-to-gross ratio, defined as the proportion of sandstone thickness within a given stratigraphic unit, serves as an effective indicator of sediment provenance. Particularly under conditions of similar A/S ratios, the net-to-gross ratio exhibits a consistent variation trend along the direction of sediment transport.

During the N1III sedimentary period, the net-to-gross ratio of the reservoir generally decreased from the periphery towards the central and eastern parts (Fig. 5D). This trend is consistent with the provenance directions indicated by the distribution of stable heavy minerals, which point towards the northwest, southwest, west, south, and northeast. In contrast, the provenance from the southeast direction was not prominently indicated. During this period, the accommodation space was relatively small, while the sediment supply from the provenance areas was sufficient. However, the sediment supply from the southeast direction was relatively weak and, influenced by stronger provenances from other directions, its contribution was not significant.

Fig. 5

Net-to-gross ratio contour maps of the K₁n₁ interval in the X Sag. (A) N10, (B) N1I, (C) N1II, (D) N1III.

During the N1II sedimentary period, the net-to-gross ratio of the reservoir generally exhibited a pattern of being higher in the western and central regions, and lower in the eastern region (Fig. 5C). Along the direction of sediment transport, the net-to-gross ratio decreased, aligning well with the provenance directions from the northwest, southwest, west, southeast, and northeast as indicated by the distribution of stable heavy minerals. Moreover, the distribution pattern of the net-to-gross ratio during this period was similar to that observed during the N1III sedimentary period. Enhanced sediment supply under reduced accommodation obscured signals from the southern provenance.

During the N1I sedimentary period, the accommodation space increased while the sediment supply weakened, resulting in a higher A/S ratio. The net-to-gross ratio of the reservoir exhibited a pattern of being higher in the western and northern regions, and lower in the eastern region (Fig. 5B).

During the N10 sedimentary period, the accommodation space continued to increase, while the sediment supply capacity further weakened, resulting in an elevated A/S ratio. The net-to-gross ratio exhibited a pattern of being higher in the western region and lower in the eastern region (Fig. 5A). These variation trends are consistent with the provenance directions indicated by heavy mineral assemblages. The southeast sector represents a sediment-underfed zone, exhibiting minimal net-to-gross values.

Paleogeomorphologic character and sediment routing systems

Paleogeomorphologic character

To reconstruct the western sediment delivery system, including provenance capacity and sediment routing pathways, during the K₁n₁ interval in the X Sag, we analyzed the basement relief west of the boundary faults. Given minimal post-K₁n₁ structural reorganization, the present-day basement topography is interpreted to represent the paleo-provenance erosion zone. Using Global Mapper, we extracted drainage divides and paleo-fluvial networks from this erosion zone (Fig. 6). Significant geomorphic contrasts between the northwestern and southeastern sectors allowed us to subdivide the area into two distinct drainage basins.

Fig. 6

Paleogeomorphic map of the western boundary fault zone of the K₁n₁ interval in the X Sag.

The geomorphology of the northwest drainage area is characterized by a topography that is lower in the west and higher in the east. The watershed is situated adjacent to the boundary fault that controls the sag. As a result, the channels to the west of the watershed are impeded and cannot transport sediments eastward. In contrast, the northeastern region receives sediment supply from the drainage area on the eastern side of the watershed, forming a proximal channels system (Fig. 6). This configuration effectively blocks westward-flowing fluvial systems, channeling sediments through paleovalleys eastward into the sag. Consequently, this watershed forms a “proximal catchment area” on the west side of the boundary fault controlling the sag.

The geomorphology of the southwest drainage area is characterized by a topography that is higher in the west and lower in the east. The watershed is situated on the western side of the southwest drainage area. A significant elevation difference exists between the eastern and western sides of this geomorphic unit within the southwest drainage area. The channels on the eastern side are characterized by their extensive area and high density. These observations indicate that the southwest drainage area represents a large provenance denudation zone with a robust sediment supply capacity.

Sediment routing systems

In this study, based on the 3D seismic data of the study area, three paleovalleys (V1, V2, and V3) were identified in the western part of the study area (Fig. 7). These paleovalleys formed during periods of rapid base-level decline in the steep-slope areas of the lacustrine basin. They represent sediment transportation channels created by the erosion of pre-deposited strata surfaces by rivers and other erosive agents. The seismic reflections of these paleovalleys typically exhibit a concave-bottom pattern with bidirectional internal filling. Locally, chaotic reflection characteristics can also be observed.

Fig. 7

Seismic reflection profiles of the K₁n₁ Strata in the Tamtsag Basin.

The cross-sectional area of paleovalleys is indicative of their sediment transportation capacity. Seismic profiles reveal that the three paleovalleys (V1, V2, and V3) developed successively. Specifically, V1 and V3 developed during the sedimentary period from N10 to N1III, while V2 developed during the sedimentary period from N10 to N1II. Based on their cross-sectional areas, the sediment transportation capacities of the paleovalleys are ranked as follows: V1 > V3 > V2.

Sedimentological and morphological analysis

Sedimentological analysis

Seismic, logging, and core data collectively indicate that three distinct sedimentary environments have developed within the K₁n₁ interval of Sag X: the fan delta front subfacies, lacustrine facies, and braided delta front subfacies.

Fan delta front subfacies

The studied lithologic assemblage is predominantly composed of gray tuffaceous siltstone, gray tuffaceous sandy conglomerate, and gray to grayish-green mudstone. Core observations from eight representative wells (including Well W7-6) reveal well-developed sedimentary structures characteristic of high-energy depositional environments, such as parallel bedding, cross-bedding, and massive bedding. Distinct scour surfaces at the base of subaqueous distributary channels provide compelling evidence of paleocurrent activity. Log facies exhibit typical box-shaped and finger-shaped morphologies, with subordinate funnel-shaped configurations. Seismic reflection patterns display medium-frequency, low-amplitude waveforms with moderate-to-low continuity, demonstrating characteristic moderate-angle progradational infilling geometries (Fig. 8).

Fig. 8

Sedimentary characteristics of the K₁n₁ interval in X sag, Tamtsag Basin.

The depositional environment is interpreted as a fan-delta front subfacies. The coarser-grained lithologies and sedimentary structures, particularly the subaqueous distributary channel scour surfaces, collectively indicate high-energy hydrodynamic conditions within this depositional system. These sedimentary features align with the depositional mechanisms described by Li et al. (2024) for rift basin settings, where fan deltas transport coarse-grained sediments into lacustrine environments⁵⁴. The thick-bedded box-shaped sandy conglomerates and siltstones represent subaqueous distributary channel microfacies, while the finger-shaped siltstones and silty mudstones correspond to sheet sands. The interbedded thin mudstone layers are interpreted as interdistributary bay deposits, with the funnel-shaped silty mudstones characteristic of mouth bar accumulations.

Lacustrine facies

The lithologic assemblage is predominantly composed of mudstone, tuffaceous siltstone, and tuffaceous fine sandstone, characterized by grayish-brown, grayish-green, and grayish-brown coloration. Core analysis reveals sedimentary structures such as massive bedding, horizontal laminations, wavy laminations, and mudstone rip-up clasts. Wireline log signatures exhibit serrated linear, serrated funnel-shaped, and serrated box-shaped patterns in gamma-ray (GR) and resistivity curves. Seismic reflection configurations display high-frequency, moderate-to-strong amplitude waveforms with parallel to subparallel reflection geometries.

The depositional environment is interpreted as lacustrine facies, further subdivided into deep-semi-deep lacustrine and shore-shallow lacustrine subfacies. The presence of wavy laminations suggests wave-modified depositional processes, while horizontal laminations indicate low-energy hydrodynamic conditions during sediment accumulation. This interpretation is corroborated by the characteristic high-frequency, parallel to subparallel seismic facies. In the Well W25-3, horizontally laminated dark gray mudstones represent quiescent water deposition, interpreted as deep-semi-deep lake subfacies. Thick box-shaped log motifs of tuffaceous siltstone correspond to lacustrine bar deposits. The presence of conglomerate fragments in core samples from the Well W35-2 indicates significant wave reworking. The serrated funnel-shaped and thin box-shaped siltstone units in the Well W35-1 are diagnostic of beach sand.

Braided delta front subfacies

The lithologic suite in the Well W82 is predominantly composed of green mudstones and gray fine sandstones, with subordinate grayish-white siltstones and gray argillaceous siltstones. Wireline log motifs display serrated thin box-shaped, medium-amplitude bell-shaped, and serrated funnel-shaped patterns. Seismic facies characterization reveals medium-to-low frequency waveforms with moderate-to-weak amplitudes, exhibiting subparallel reflection configurations with low-angle progradational infill features. These seismic units display sheet-like to weakly wedge-shaped external geometries, predominantly distributed within the gentle slope area of the study region.

This depositional system, located in the gentle slope zone, is interpreted as a braided delta front subfacies. The thin box-shaped log patterns of fine sandstones represent subaqueous distributary channel microfacies, where serrated curve features indicate significant wave-modification processes. Serrated funnel-shaped siltstone units correspond to mouth bars, while bell-shaped siltstone motifs are diagnostic of sheet sand accumulation.

Characterization of sedimentary microfacies distribution based on seismic RGB attributes

As previously analyzed, four subfacies and twelve microfacies are developed within the study area. Within the fan-delta front subfacies, four microfacies are identified: subaqueous distributary channels, mouth bars, sheet sands, and subaqueous distributary interchannel areas. Within the shore-shallow lacustrine subfacies, three microfacies are identified: beach sands, bar sands, and shore-shallow lacustrine mudstones. Within the braided delta front subfacies, four microfacies are identified: subaqueous distributary channels, mouth bars, sheet sands, and subaqueous distributary interchannel areas. Within the deep-semi-deep lacustrine subfacies, the deep-semi-deep lacustrine mudstone microfacies is identified.

RGB seismic attribute slices, generated by assigning different colors to seismic volumes of varying frequencies, effectively highlight geological body boundaries through color contrasts and brightness variations. This technique has been widely applied in sedimentary system identification and reservoir prediction. In this study, seismic data volumes corresponding to 40 Hz (low frequency), 70 Hz (medium frequency), and 100 Hz (high frequency) were extracted. The 40 Hz volume was assigned blue, the 70 Hz volume green, and the 100 Hz volume red. These three frequency volumes were then fused to generate four RGB attribute slices along the target horizon (Fig. 9). This method leverages the unique characteristics of different frequency responses to enhance the visibility of geological features, making it particularly effective for identifying sedimentary facies and reservoir boundaries.

Based on seismic reflection characteristics, well log responses, and geomorphological features, the depositional environments interpreted from the RGB slices are as follows (Figs. 9 and 10):

Fig. 9

RGB seismic attribute slices of the K₁n₁ in X sag, Tamtsag Basin. (A) N10, (B) N1I, (C) N1II, (D) N1III. The yellow dashed line in each panel delineates the extent of the lobe body.

Fig. 10

Distribution map of sedimentary microfacies of the K₁n₁ in X sag, Tamtsag Basin. (A) N10, (B) N1I, (C) N1II, (D) N1III.

• (1) During the N1III deposition, RGB attribute slices reveal distinct depositional environments. The deep blue and orange lobes adjacent to the western boundary fault represent the fan delta front subfacies. The yellow lobes in the south, along with the bright blue lobes in the southeast and northeast, represent the braided delta front subfacies.

• (2) During the N1II deposition, RGB attribute slices reveal distinct depositional environments. The bright pink lobes in the west represent the fan delta front subfacies. The light blue lobes in the south, the bluish-purple lobes in the southeast, and the blue-green lobes in the northeast represent the braided delta front subfacies.

• (3) During the N1I deposition, RGB attribute slices reveal distinct depositional environments. The bluish-purple lobes adjacent to the western boundary fault represent the fan delta front subfacies. The green lobes in the south, along with the yellowish-green lobes in the southeast and northeast, represent the braided delta front subfacies.

• (4) During the N10 deposition, RGB attribute slices reveal distinct depositional environments. The yellowish-pink lobes adjacent to the western boundary fault represent the fan delta front subfacies. The dark blue lobes in the south and the blue-green lobes in the northeast represent the braided delta front subfacies. During the N10 deposition, RGB attribute slices reveal distinct depositional environments. The yellowish-pink lobes adjacent to the western boundary fault represent the fan delta front subfacies. The dark blue lobes in the south and the blue-green lobes in the northeast represent the braided delta front subfacies.

Discussion

Control effect of provenance supply on the combination pattern of the S2S

The central part of the study area is predominantly characterized by the shore-shallow lacustrine subfacies, specifically beach sands and bar sands. The fan-delta front lobate bodies in the western part of the study area are sourced from the transverse western catchment area of the depression and show a good correspondence with paleovalleys. The differential activity of the western boundary fault during different periods caused the topography in the transverse western catchment area to steepen, providing favorable conditions for the formation of paleovalleys. Owing to the relatively stronger sediment supply capacity of the western catchment area, sediments were reworked and transported by waves and currents, accumulating parallel to the shoreline in the central depression and exhibiting a sheet-like distribution. Within this, bar sands are primarily distributed internally within the beach sands (Fig. 10).

During the N1III depositional period, the sediment supply capacity of each catchment area was relatively limited. In the western part of the study area, three isolated fan-delta lobes developed, while one braided delta lobe developed in each of the south, southwest, and northwest. The northwestern and southwestern catchment areas, located transverse to the short axis of the X sag and in close proximity to the study area, resulted in short sediment transport distances and relatively strong sediment supply capacities. Consequently, the fan-delta lobes in these areas were relatively large in area. The northeastern and southeastern catchment areas are also oriented transverse to the short axis, suggesting relatively short sediment transport distances, their sediment supply capacities were limited. Consequently, the braided delta lobes sourced from these areas were relatively small in area. In contrast, the southern catchment area, located along the axial direction of the X sag, has longer sediment transport distances. Despite this, the braided delta lobes sourced from the southern catchment are larger in scale than those from the northeastern and southeastern catchments. This indicates that the sediment supply capacity of the axial southern catchment in the X sag is stronger than that of the transverse northeastern and southeastern catchments.

During the N1II depositional period, the sediment supply capacity of all catchment areas increased significantly. As a result, both the fan-delta lobes and braided delta lobes expanded notably in area. Moreover, the fan-delta lobes transitioned from isolated “lobate” shapes to interconnected “apron-like” configurations.

During the N1I depositional period, delta lobes continued to evolve from the N1II interval. However, a reduction in sediment supply capacity from all catchment areas led to a decrease in the areas of the western fan-delta lobes, the southern braided delta lobe, the southeastern braided delta lobe, and the northeastern braided delta lobe.

During the N10 depositional period, the sediment supply capacity of all catchment areas further diminished. Consequently, the area of the western fan-delta lobes decreased, and their distribution pattern transitioned from an early continuous “apron-like” configuration to three isolated “lobate” forms. The area of the northeastern braided delta also decreased, and the southeastern braided delta lobe disappeared. In contrast, the area of the southern braided delta lobe increased. However, its thickness became thinner, resulting in a decrease in volume and thus a smaller depositional scale.

Influence of evolution of boundary faults on the migration of sedimentary-subsidence center

The influence of structure on the sedimentary filling of sand bodies is primarily mediated through changes in accommodation space and the control of sediment transport pathways^55,56. In this study, we elucidate the impact of boundary-controlling faults on sediment filling by analyzing the relationship between the growth and evolution of these faults and the depocenters and subsidence centers during different periods (Fig. 11).

Fig. 11

Thickness Distribution of Sand Bodies in the K₁n₁ Interval of Sag X, Tamtsag Basin. (A) N10, (B) N1I, (C) N1II, (D) N1III.

From the perspective of boundary fault growth processes, both F1 and F2 initiated during the N1II. As strata deposition progressed, the activity of these faults gradually increased, and their lengths progressively extended. Specifically, during the K₁n₁ interval, F2 manifested as isolated fault segments, while F1 exhibited characteristics of segmented growth.

During the N1III sedimentary period, the sedimentation center was situated in the western part of the study area, whereas the subsidence centers were located in the northwestern and eastern parts. At this time, the boundary faults F1 and F2 had not yet initiated.

During the N1II sedimentary period, the boundary faults F1 and F2 initiated. F1 was characterized by two isolated fault segments. A sedimentation center developed in the down-dip direction of F1-1, while sedimentation-subsidence centers formed in the down-dip directions of F1-2 and F2. The eastern subsidence center exhibited an inherited development relationship with the N1III period.

During the N1I sedimentary period, three sedimentation centers and one subsidence center developed within the study area. Each sedimentation center exhibited an inherited development relationship with those from the N1II sedimentary period. As the activity of faults F1-1, F1-2, and F2 increased, the distribution ranges of the sedimentation centers correspondingly expanded.

During the N10 sedimentary period, the activity of boundary faults further intensified, with each fault segment experiencing an increase in both length and throw. Two sedimentation centers and two subsidence centers developed, nearly coinciding spatially. The southern sedimentation-subsidence center was situated in the down-dip direction of F1-1, while the northern sedimentation-subsidence center was located between F1-2 and F2. By the end of the N10 sedimentary period, F1-1 and F1-2 had connected to form a major fault, F1, and F1-2 had rigidly linked with F2. The distribution pattern of the boundary sag-controlling faults at this stage became similar to the present-day fault morphology.

These characteristics indicate that the intensified activity of the boundary sag-controlling faults led to a gradual increase in the length and throw of these faults. Concurrently, the sedimentation-subsidence centers migrated progressively westward, ultimately resulting in a near-coincidence of these centers.

Evolution of the S2S of the K₁n₁ of X sag, Tamtsag basin

Developing S2S models that are specifically adapted to unique geological settings aids in clarifying the distribution patterns of sand bodies. In the K₁n₁ interval of the X sag, this study categorizes the S2S systems into three distinct types based on catchment distribution and sand body genetic types: the “Transverse Drainage–Fan Delta” S2S system, the “Transverse Drainage–Braided Delta” S2S system, and the “Axial Drainage–Braided Delta” S2S system (Fig. 12).

Fig. 12

The S2S Coupling Model of the K₁n₁ of the X sag, the Tamtsag Basin.

During the N1III sedimentary period, the study area was supplied by five distinct catchment areas. The “transverse drainage–fan delta” S2S system developed in the western part of the study area, the “transverse drainage–braided delta” S2S system developed in the northeast and southeast, and the “axial drainage–braided delta” S2S system developed in the southern part. Within the “transverse drainage–fan delta” S2S system, three fan-delta lobe bodies are present, with paleovalleys on the steep slopes acting as critical sediment transport pathways (Fig. 8). In the “transverse drainage–braided delta” S2S system, the primary sediment transport channels are gentle slope channels, and the sedimentary system is predominantly characterized by the short-axis braided delta front subfacies. In the “axial drainage–braided delta” S2S system, the main sediment transport system is the axial channel, and the developed sedimentary system primarily consists of the long-axis braided delta front subfacies.

During the N1II sedimentary period, the S2S systems in the study area evolved in an inherited manner from those of the N1III period, with three paleovalleys forming adjacent to the boundary sag-controlling faults. The enhanced sediment supply capacity of the catchment areas during this period led to a significant expansion of the delta lobe distribution area. Specifically, the previously “isolated” fan-delta lobes gradually coalesced to form an “apron-like” configuration (Fig. 10). Concurrently, the initiation of activity in the boundary sag-controlling faults resulted in a gradual westward migration of the sedimentation-subsidence center.

During the N1I depositional period, the S2S systems continued to evolve in an inherited manner from the preceding N1II interval. The sediment supply capacity from the catchment areas diminished, leading to a reduction in the volumes of both fan-delta lobes and braided river delta lobes. Concurrently, the intensified activity of the boundary-controlling faults resulted in two significant changes: (1) the spatial expansion of the depocenter and the western subsidence center, and (2) the disappearance of the eastern subsidence center.

During the N10 sedimentary period, the sediment supply capacity from the catchment areas diminished. The study area received sediments from the northwestern, southwestern, northeastern, and southern catchment areas. Delta lobes in the southeastern part disappeared, while the volumes of braided delta lobes in the southern and northeastern parts, as well as fan delta lobes adjacent to the boundary faults, decreased. The fan delta lobes contracted from their original contiguous “apron-like” configuration to isolated lobes (Fig. 10). Activity along the boundary faults peaked during the N10 sedimentary period. By the end of the N10 period, faults F1-1 and F1-2 coalesced into a single fault and rigidly connected with F2 (Fig. 2). The southern sedimentation-subsidence center was situated in the down-dip direction of F1-1, while the northern sedimentation-subsidence center was located at the junction of F1-2 and F2.

Conclusion

This study investigates the S2S system of the K₁n₁ interval within the X sag, Tamtsag Basin. An integrated approach, incorporating fault-displacement backstripping, sedimentological analysis, and seismic geomorphology, was employed. This methodology utilizes sediment petrology, well-log data, and seismic characteristics. The key conclusions are as follows:

• (1) Elemental analysis of the S2S system indicates that the northwestern and southwestern catchments served as the primary sediment sources, supplemented by the northeastern, southeastern, and southern catchments. During the K₁n₁ depositional period, the northeastern, southeastern, and southern catchments exhibited lower sediment supply capacity. Sediments were predominantly transported via subaqueous distributary channels within braided delta front subfacies on gentle slopes. In contrast, the western catchment displayed higher sediment supply capacity, with sediments transported via paleovalleys in fan delta front subfacies along steep slopes. Consequently, reservoir sand bodies within fan deltas adjacent to the western boundary-controlling fault were significantly larger-scale compared to braided delta reservoirs sourced from the other catchments.

• (2) The characteristics of the S2S system in the K₁n₁ interval of the X sag are predominantly governed by the growth and evolution of the west-boundary sag-controlling faults and the sediment supply capacity of the catchment areas. ① During the N1III sedimentary period: The west-boundary sag-controlling faults were undeveloped, resulting in low sediment supply from the western catchment. Consequently, the scale of the western fan delta lobes remained relatively small. The depocenter developed primarily in the western part of the study area, while the subsidence center was regionally located in the eastern and northwestern regions. ② During the N1II sedimentary period: Reactivation of the west-boundary sag-controlling faults enhanced sediment supply from the western catchment, thereby increasing the scale of the fan delta lobes. Inherited subsidence persisted in the eastern depocenter, while multiple depocenters developed within the western part of the basin. ③ During the N1I sedimentary period: Enhanced activity of the west-boundary sag-controlling faults coincided with reduced sediment supply from the western catchment. Concurrently, the scale of the sag-controlling faults increased, whereas the dimensions of the fan delta lobes decreased. The eastern subsidence center migrated or ceased to exist as a distinct feature, while inherited development persisted in the western depocenters. ④ N10 Sedimentary Period: The western boundary faultsreached peak activity, accompanied by a further reduction in sediment supply from the western provenance. Fan-delta lobe development diminished to its minimum scale. The geometry of these faults at this stage approximates their present-day configuration, and the sedimentation-subsidence center migrated towards their hanging wall.

• (3) Based on the developmental characteristics and primary controlling factors of the S2S system elements, three distinct S2S coupling models have been established: ① the “transverse drainage–fan delta” model, ② the “transverse drainage–braided delta” model, and ③ the “axial drainage–braided delta” model. The “transverse drainage–fan delta” model: This S2S system is primarily sourced from the northwestern and southwestern provenance areas. Sediments are transported via steep slopes and paleovalleys, forming fan delta front deposits and beach-bar complexes within shallow lake subfacies. The “transverse drainage–braided delta” model: This system is mainly supplied by the northeastern and southeastern provenance areas. Sediments are transported through gentle slope channels, developing short-axis braided delta front deposits. The “axial drainage–braided delta” model: Predominantly sourced from the southern provenance area, sediments in this system are primarily transported through the axial river channel, forming elongate braided delta front deposits.